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Thread: mystery vent in bathroom floor above crawl space

  1. #1
    DIY Junior Member allycat's Avatar
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    Oct 2004

    Default mystery vent in bathroom floor above crawl space


    We have a new-to-us log cabin in central Maine. It is 20 years old, built on a crawl space which has a plywood skirt around it, with blue foam insulation inside the crawl walls. No insulation under the floor. The crawl is about 3 feet deep. All plumbing is in the right corner of the house: full bath downstairs that also houses the hot water heater, kitchen sink in adjacent room, 1/2 bath upstairs right above the downstairs bath. Heat is propane by 3 rinnai heaters: 2 in main downstairs living space and one small one that was recently installed in the downstairs bathroom.

    We plan to spend some weekends here in winter and thus leave the heat at about 60 while we're not here. We shut the water off when we're gone and have an automatic generator.

    Plumbing consists of a well line coming in to the plumbed corner of the house, via the crawl space. The crawl space is accessed through a trap door in the center of the bathroom floor. About a foot away from the trap door is a vent covering another hole in the flloor, about 4 x 6 inches, and that hole sits about 6 inches outslde of the hot water heater closet. The previous owner, who lived here year-round, had installed a little fan in the hole under that vent. We thought this might have been done to draw heat into the space during cold snaps but his widow now tells us she thinks it was to avoid moisture down there during one humid summer. Neither theory makes much sense to us.

    Also, we are discovering that the Rinnai in that room cycles over and over, even at 60, and it seems like the heater may be drawing cold air up from that flloor vent as it runs. The heater is only about 2 feet from the vent

    Any ideas what this vent is there to accomplish and whether it should be blocked off for the winter?

    Many thanks for any input.

  2. #2
    DIY Senior Member Reach4's Avatar
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    Sep 2013


    I think you should consider putting down a vapor barrier on the ground in the crawl space. Maybe 12 mil polyethylene. I don't know much about doing that, but I suggest you search for info on that.

    I expect the vent was put in for the reason that the wife said. I doubt it was all that effective for the purpose.

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